THURSDAY, May 31 (HealthDay News) -- As many as 2.2 million people in the United States may be infected with chronic hepatitis B virus, a new study suggests.

Most of those infected come from countries in Asia and Africa, where regular vaccination for the virus has not been routine, researchers report.

"Hepatitis B is a common infection transmitted at birth or in early childhood. When it's transmitted at that young an age, it tends to remain a chronic infection," said Dr. John Ward, director of the viral hepatitis program at the U.S. Centers for Disease Control and Prevention and co-author of an editorial accompanying the study.

The populations who have the infection are diverse, he said. "This illustrates the difficulty we have in developing prevention programs that take into account cultural and language differences," Ward noted.

In the United States, infants started being vaccinated for the virus in the early 1990s and the rate of vaccination is now over 90 percent, Ward explained. So, the rate of the infection among those born in the United States has dropped significantly, he said.

It is only in the last 10 years that efforts have been under way to vaccinate infants in other countries against hepatitis B, Ward added.

Many people with the virus are not aware that they are infected and run the risk of giving it to others and becoming sick themselves, he added.

There are effective treatments available using antiviral medicines. Often, these drugs have to be taken for an extended time, but when successful, the damage to the liver can be reversed, Ward said. "It's a very effective treatment," he pointed out.

If untreated, people can develop liver cancer, which is the third-leading cause of death around the world, Ward said.

People who come to the United States from areas where hepatitis B is common should be tested for the virus so they can receive treatment, he suggested.

The report will be published in the July print issue of the journal Hepatology.

Hepatitis B affects as many as 400 million people around the world. If left untreated, up to 25 percent of those infected are at risk of dying from liver cancer or liver disease.

In 2006, the CDC estimated that some 800,000 to 1.4 million people in the United States had the virus.

For this study, a research team led by Dr. Carol Brosgart, a member of the faculty at the division of global health at the University of California, San Francisco, and senior advisor on science and policy to the Viral Hepatitis Action Coalition at the CDC Foundation, looked at all the medical literature on the prevalence of hepatitis B around the world from 1980 to 2010.

That analysis revealed that in 2009, between 1.04 million and 1.61 million people born abroad who were living in the United States had chronic hepatitis B.

Those infected came mostly from Asia, Africa and Central America, accounting for 58 percent, 11 percent and 7 percent of cases, respectively, which is much higher than previously thought, the researchers noted.

"This study highlights an important health concern for the U.S. and the need for broader hepatitis B screening of foreign-born individuals," Brosgart said in a statement. "Given our ability to treat chronic hepatitis B virus and to monitor for emergence of liver cancer when it is treatable, physicians should screen the foreign-born, their children and close contacts."

Another expert, Dr. Marc Siegel, an associate professor of medicine at NYU Langone Medical Center in New York City, added that "we have an epidemic of hepatitis B around the world and we have to be alert to it in people who move here from other countries."

In addition, its prevalence in the United States has been underestimated, as this study confirms, he said.

It's easy to get the virus, which is usually transmitted sexually or through contact with infected blood or among injection drug users, he said.

The epidemic of hepatitis B has led to an epidemic of liver cancer around the world, Siegel added.